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It Can Make You Cool or Make You Miserable

Usually sweat just disappears, cooling the skin as it goes. But it can also leave smelly, salty, drug-filled footprints behind, and sometimes it flows so copiously it can ruin lives and careers.

Two different kinds of glands contribute to the sweat pool. The eccrine sweat glands (about 75 percent of the total), located everywhere on the skin but the lips and a few other small sites, begin working at birth and release a clear, watery fluid onto the skin.

The apocrine glands, in the armpit and groin, begin working in the teenage years, and their fluid is milky and viscous. Skin bacteria thrive in apocrine sweat, and byproducts of bacterial metabolism make the odor. Deodorants abolish it with perfumes and antiseptics, while the chemicals in antiperspirants block the mouth of the gland and prevent sweat release to the skin.

Our armpits may be the focus of much self-consciousness and advertisement, but they contribute less than 1 percent of total sweat: one study (sponsored by antiperspirant manufacturer Unilever) found that antiperspirant applied to the armpits of athletes made no difference at all to their body cooling patterns: the odor-free group did not overheat.

Eccrine sweat is both simpler and more complex than apocrine. The sweat itself consists mostly of water and a little salt. The palms and the soles have more eccrine glands than the rest of the body, and sweat there is triggered by emotion as well as heat. Anthropologists suggest that this pattern may have evolved to help primitive man’s grip and traction while hunting and fighting.

Athletes lose enough salt in their sweat to require close attention to replacing it, but for the average sedentary human, even in hot climates, sweat salt loss is quite small.

The important exceptions to this rule are people with cystic fibrosis, the genetic disease in which a defective cell protein causes abnormalities in many body fluids. Among them is sweat: the disease disables the pump that recaptures salt as sweat moves from the base of the gland out to the skin’s surface. Without processing by this pump, sweat has a salt concentration similar to blood’s.

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Even before a child gets a formal diagnosis of cystic fibrosis with a rigorous sweat test, parents may unconsciously make the diagnosis with their salty-tasting kisses.

Using sweat to diagnose other diseases is still a science in its infancy. Unlike blood, urine and saliva, pure fresh sweat contains no microorganisms or cells. It cannot transmit infections like H.I.V., and it gives few clues to the nature of any internal problems. It does, though, often contain traces of the same proteins and drugs that pass through in the bloodstream.

Thus, scientists have diagnosed hepatitis B by the presence of a characteristic viral protein in sweat. They have found alcohol, caffeine, nicotine, cocaine, amphetamines and other drugs in sweat. A controversial “drugs of abuse” sweat patch has been on the market since 1995: a gauze pad taped to the skin collects a week’s worth of sweat for drug testing. Proponents claim it is less subject to tampering than urine tests, while critics maintain it is also less reliable.

Sometimes, though, the sweat flows all too freely: a condition called hyperhidrosis afflicts an estimated 3 percent of the United States population with uncontrollable sweating, usually from the hands, feet or armpits.

“It can cause quite a lot of misery,” said Dr. Gil Yosipovitch, a dermatologist at Wake Forest University, recalling teenage patients, for example, who have been penalized during tests because their pencils keep slithering to the floor.

Some cases can be managed with heavy-duty antiperspirants, but others may require injections of Botox, which paralyzes the nerves leading to the sweat glands, or even surgery on the nerves.

A version of this article appears in print on , on Page F6 of the New York edition with the headline: It Can Make You Cool or Make You Miserable. Order Reprints|Today's Paper|Subscribe